The invention concerns an aluminium casting alloy, in particular an aluminium diecasting alloy.
Diecasting technology has today developed until it is possible to produce castings with high quality standards. The quality of a diecasting depends, however, not only on the machine setting and the process selected but to a great extent also on the chemical composition and grain structure of the casting alloy used. The latter two parameters are known to influence the castability, the supply behaviour (G. Schindelbauer, J. Czikel "Mould Filling Capacity and Volume Deficit of Conventional Aluminium Diecasting Alloys", Casting Research 42, 1990, p. 88/89), the mechanical properties and--what is particularly important in diecasting--the life of the casting tools (L.A. Norstr.div.m, B. Klarenf jord, M. Svenson "General Aspects on Wash-out Mechanism in Aluminium Diecasting Dies", 17th International NADCA Diecasting Congress 1993, Cleveland Ohio).
In the past, little attention was paid to the development of alloys suitable in particular for the high quality castings required for diecasting. Most efforts were devoted to the further development of the process technology of the diecasting process. However, designers in the automobile industry in particular are being called upon more and more often to produce weldable components of high ductility in the diecasting process as diecasting is the most cost-favourable production method for large quantities.
The further development of diecasting technology has made it possible today to produce weldable and heat-treatable castings of high quality. This has expanded the area of application for diecastings to safety-relevant components. Usually for such components, AlSiMg alloys are used today as these have good casting properties with low mould wear. In order for the required mechanical properties, in particular a high elongation at yield, to be achieved, the castings must be subjected to heat treatment. This heat treatment is necessary to coalesce the casting phase and thus achieve a tough yield behaviour. Heat treatment normally means solution treatment at temperatures just below the solidus temperature with subsequent quenching in water or another medium to temperatures&lt;100.degree. C. The material treated in this way now has a low limit of elasticity and tensile strength. In order to raise these properties to the required value, artificial ageing is then carried out. This can also be process-related, e.g. by thermal shock on painting or stress-relieving annealing of an entire component group.
As diecastings are cast close to the final dimensions, they usually have a complex geometry with low wall thicknesses. During the solution treatment, and in particular in the quenching process, distortion must be expected which can require retouching, e.g. straightening of the casting, or in the worst case can lead to rejection. Solution treatment also incurs additional costs and the economic benefits of this production method could be substantially improved if alloys were available which fulfilled the required properties without heat treatment.
AlMg alloys are known which are characterized by a high ductility. Such an alloy is disclosed for example in US-A-5 573 606. However, these alloys have the disadvantage of a high mould wear and cause problems on mould removal, which considerably reduces productivity.